Flame retardants play an important role in safeguarding life and property. Their use in flexible polyurethane foam is mandated by a considerable number of regulations specifying fire safety requirements. In addition, the Consumer Products Safety Commission (CPSC) in the United States is considering a regulation for upholstered furniture that would extend a standard similar to or more stringent than the existing California TB 117 standard. Further, recent legislation proposed in California and other states would restrict use of chlorinated and brominated flame retardants. As a result, more efficient and sustainable flame retardants, both halogen-free and halogenated, continue to be sought for use in flexible polyurethane foams.
In flexible polyurethane foams, it is generally not possible to simply substitute one flame retardant for another since flame retardants interact in very specific ways with the polymer matrix and may affect properties of the product. Thus performance is a key factor in substitution. An acceptable flame retardant must not only meet flammability standards but physical properties requirements including resistance to discoloration as well.
In the manufacture of flexible foams, large buns of foam are produced and then set aside to cure. Temperatures within the bun can reach 150° to 180° C. or higher. The insulating properties of the foam maintain this temperature in the interior of the bun for an extended period of time. At such high temperatures some of the foam forming reactions are reversible.
The flame retardant should be able to withstand high temperatures and not contribute to foam degradation usually visible as scorching or charring in the center of the bun.
For low density foams the exotherm generated during the preparation of the foam increases, due to the high amount of added water (resulting in high heat evolution in its reaction with isocyanate to form CO2) needed to lower the density. This makes low density foams particularly sensitive to scorch. High susceptibility to scorch and particularly high flammability of low density foams requires development of robust and highly efficient flame retardants.
Low density flexible polyurethane foams are particularly difficult to flame retard. In a low density flexible polyurethane foam formulation, a small amount of high surface polymer film is surrounded by air facilitating rapid combustion. One solution to this problem has been the use of relatively high amounts of flame retardants to offset their comparatively low flame retardancy effectiveness. This solution, while technically effective in reducing flammability, can have detrimental effects on physical properties of the foam such as flexibility, average cell size, open cell content and/or color stability (resistance to scorch).
At first glance, one possible solution to the problem of imparting required levels of flame retardancy performance to flexible polyurethane foams while retaining acceptable foam properties would be the use of one or more organophosphorus flame retardants having a relatively high content of phosphorus, e.g., at least 10 weight percent phosphorus, for optimum flame retardancy effectiveness. This approach would seem to offer the advantage of achieving required levels of flame retardancy with reduced amounts of flame retardant compound(s) while reducing or minimizing any negative impact on foam properties. Molecules having high phosphorus contents are either oligomers or polymers with multiple phosphorus atoms or small molecules with a single phosphorus atom. Oligomeric and polymeric compounds are usually solids or very viscous liquids and are therefore difficult and/or impractical to use in polyurethane formulations.
However, the use of small high phosphorus content flame retardant molecules is also subject to disadvantages, especially evident in those molecules that are chemically inert relative to the polyurethane foam-forming composition in which they are incorporated, due to their being fairly volatile and as a result, exhibiting a tendency to progressively evaporate from polyurethane foams in which they are physically entrained and/or rapidly escape from the foams ahead of an advancing flame front to which the foams are exposed, both outcomes negatively affecting the flame retardancy effectiveness of these types of molecules.
By way of avoiding the aforementioned drawbacks associated with chemically inert volatile organophosphorus flame retardants, compounds possessing hydroxyl functionality, i.e., so-called reactive flame retardants, have been utilized. During the polyurethane foam-forming reaction, the hydroxyl group(s) of such organophosphorus flame retardant compounds will react with isocyanate groups of the polyisocyanate component(s) of the foam-forming composition thereby chemically anchoring the flame retardants to the foam matrix except when the foam is exposed to flame.
Mixtures of hydroxyl-containing organophosphorus compounds for use as flame retardants in polyurethane foams are known from U.S. Pat. Nos. 3,515,776 and 3,600,339. As a result of the process by which they are made, the mixtures of hydroxyl-containing organophosphorus compounds described in each of these patents contain both a monohydroxy cyclic phosphonate and at least one polyhydroxy phosphonate which may be a cyclic or non-cyclic phosphonate. These mixtures of monohydroxy and polyhydroxy organophosphorus flame retardants, due to the presence of the polyhydroxy phosphonate component(s) therein, will possess a higher hydroxyl number (a measure of the concentration of hydroxyl groups in a substance) than an equal weight amount of monohydroxy cyclic phosphonate which is substantially free of polyhydroxy phosphonate(s). As recognized in US 2007/0112084, high hydroxyl numbers of a reactive flame retardant composition are apt to be disadvantageous in that obtaining a flame retardant polyurethane foam of acceptable properties with them requires a careful balancing between the various components of the foam-forming composition, a requirement that is said to necessitate time-consuming development work. According to US 2007/0112084, the difficulty of obtaining a balanced foam-forming composition becomes less as the hydroxyl number of the flame retardant compound or mixture of compounds decreases and as the level of flame retardant required for satisfactory flame retardancy performance becomes smaller. For this reason, US 2007/0112084 gives preference to reactive flame retardants having a low hydroxyl number and/or high activity. Included within the genus of monohydroxy phosphonate flame retardant compounds described in US 2007/0112084 are those in which a hydroxyalkyl group of at least two carbon atoms is directly bonded to the phosphorus atom of a six-membered ring.
It has now been discovered that a particular class of monohydroxy cyclic phosphonates which is substantially free of polyhydroxy phosphonate, and featuring a single hydroxylalkyl group bonded to a ring carbon atom in contrast to a ring phosphorus atom as in the flame retardants of US 2007/0112084, provides flexible polyurethane foams meeting the strictest standards of flame retardancy performance while still exhibiting industry-acceptable foam properties.